Re: [PATCH 07/33] autonuma: mm_autonuma and task_autonuma data structures

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Oct 11, 2012 at 07:15:20PM +0200, Andrea Arcangeli wrote:
> On Thu, Oct 11, 2012 at 01:28:27PM +0100, Mel Gorman wrote:
> > s/togehter/together/
> 
> Fixed.
> 
> > 
> > > + * knumad_scan structure.
> > > + */
> > > +struct mm_autonuma {
> > 
> > Nit but this is very similar in principle to mm_slot for transparent
> > huge pages. It might be worth renaming both to mm_thp_slot and
> > mm_autonuma_slot to set the expectation they are very similar in nature.
> > Could potentially be made generic but probably overkill.
> 
> Agreed. A plain rename to mm_autonuma_slot would have the only cons of
> making some code spill over 80 col ;).
> 

Fair enough :)

> > > +	/* link for knuma_scand's list of mm structures to scan */
> > > +	struct list_head mm_node;
> > > +	/* Pointer to associated mm structure */
> > > +	struct mm_struct *mm;
> > > +
> > > +	/*
> > > +	 * Zeroed from here during allocation, check
> > > +	 * mm_autonuma_reset() if you alter the below.
> > > +	 */
> > > +
> > > +	/*
> > > +	 * Pass counter for this mm. This exist only to be able to
> > > +	 * tell when it's time to apply the exponential backoff on the
> > > +	 * task_autonuma statistics.
> > > +	 */
> > > +	unsigned long mm_numa_fault_pass;
> > > +	/* Total number of pages that will trigger NUMA faults for this mm */
> > > +	unsigned long mm_numa_fault_tot;
> > > +	/* Number of pages that will trigger NUMA faults for each [nid] */
> > > +	unsigned long mm_numa_fault[0];
> > > +	/* do not add more variables here, the above array size is dynamic */
> > > +};
> > 
> > How cache hot is this structure? nodes are sharing counters in the same
> > cache lines so if updates are frequent this will bounce like a mad yoke.
> > Profiles will tell for sure but it's possible that some sort of per-cpu
> > hilarity will be necessary here in the future.
> 
> On autonuma27 this is only written by knuma_scand so it won't risk to
> bounce.
> 
> On autonuma28 however it's updated by the numa hinting page fault
> locklessy and so your concern is very real, and the cacheline bounces
> will materialize.

It will be related to the knuma_scan thing though so once every 10
seconds, we might see a sudden spike in cache conflicts. Is that
accurate? Something like perf top might detect when this happens but it
can be inferred using perf probe on the fault handler too.

> It'll cause more interconnect traffic before the
> workload converges too. I thought about that, but I wanted the
> mm_autonuma updated in real time as migration happens otherwise it
> converges more slowly if we have to wait until the next pass to bring
> mm_autonuma statistical data in sync with the migration
> activities. Converging more slowly looked worse than paying more
> cacheline bounces.
> 

You could argue that slower converging also means more cross-node
traffic so it costs either way.

> It's a tradeoff. And if it's not a good one, we can go back to
> autonuma27 mm_autonuma stat gathering method and converge slower but
> without any cacheline bouncing in the NUMA hinting page faults. At
> least it's lockless.
> 

Yep.

> > > +	unsigned long task_numa_fault_pass;
> > > +	/* Total number of eligible pages that triggered NUMA faults */
> > > +	unsigned long task_numa_fault_tot;
> > > +	/* Number of pages that triggered NUMA faults for each [nid] */
> > > +	unsigned long task_numa_fault[0];
> > > +	/* do not add more variables here, the above array size is dynamic */
> > > +};
> > > +
> > 
> > Same question about cache hotness.
> 
> Here it's per-thread, so there won't be risk of accesses interleaved
> by different CPUs.
> 

Ok thanks. With that clarification

Acked-by: Mel Gorman <mgorman@suse.de>

While I still have concerns about the cache behaviour of this the basic
intent of the structure will not change no matter how the problem is
addressed.

-- 
Mel Gorman
SUSE Labs